Convert USE_PROPRIETARY_CODECS to a buildflag header.

Convert the preprocessor define to a buildflag
//header defined in //media. The build arg is kept in //build/config/features.gni due to a dependency on WebRTC (described in a comment).

R=xhwang@chromium.org

Review-Url: https://codereview.chromium.org/2572883002 .
Cr-Commit-Position: refs/heads/master@{#439175}
46 files changed